Technical Field
The present invention relates to a process for manufacturing a microelectromechanical interaction system for a storage medium, in particular for so-called “probe storage” applications, to which the following treatment will make explicit reference, without this implying any loss of generality.
Description of the Related Art
As is known, in the last few years alternative storage systems have been proposed to overcome the limitations of traditional storage systems based on magnetism, such as, for example, hard disks. Amongst these systems, of particular importance are the so-called “probe storage” (also referred to as “atomic-level storage” or “atomic storage”) systems, which enable high data-storage capacities to be obtained in small dimensions, with low manufacturing costs.
In brief (FIG. 1), a probe-storage device 1 comprises a two-dimensional array of interaction systems (or probes) 2, fixed to a common substrate 3, for example made of silicon, in which a control electronics is provided, for example using CMOS technology. The array is arranged above a storage medium 4, typically not patterned, and is mobile relative to the storage medium, generally in a first direction x and in a second direction y, which are mutually orthogonal, by the action of a micromotor associated therewith. Each interaction system 2 comprises: a supporting element 5 made of semiconductor material, in particular silicon (generally known as “cantilever” or “cantilever beam”), suspended in cantilever fashion above the storage medium 4, and moveable in a third direction z, orthogonal to the first and second directions x, y so as to approach the storage medium 4; and an interaction element 6 (also defined as “sensor” or “contact element”), carried by the supporting element 5 at a free end thereof, and facing the storage medium 4. In particular, by the term “interaction” is meant any operation of reading, writing or erasure of a single bit (or a number of bits) of information, which implies an exchange of signals between the interaction system 2 and the storage medium 4. Via the respective interaction element 6, having nanometric dimensions, each interaction system 2 is able to interact locally at an atomic level with a portion of the storage medium 4, for writing, reading, or erasing bits of information.
The physical characteristics (hardness, roughness, etc.), morphological characteristics (dimensions, shape, etc.) and electrical characteristics (resistivity, conductivity, etc.) of the interaction element 6 are strictly correlated to the material of the storage medium 4 with which it is associated (polymeric, ferroelectric, phase-change, etc.), and to the interaction mechanisms for reading/writing/erasing data (thermal, piezoresistive, charge-transfer, etc.).
For example, storage systems of the probe-storage type have been designed, in which the interaction mechanisms involve thermal and/or piezoresistive processes. In these systems, the interaction element 6 has a sharpened shape, enabling the formation of “bits” with nanometric dimensions so as to increase storage density. In a known way, during operations of writing of data, the interaction element 6 is heated via appropriate heating elements (for example, of a resistive type) integrated in the interaction system 2, and is pushed into contact with the storage medium 4, for formation of single bits (the presence or absence of a bit encoding in a binary way the data to be stored). Reading operations are based on resistance variations occurring in the interaction system 2 as a function of temperature, or as a result of the piezoresistive effect due to mechanical deformations, when the interaction system is moved above the storage medium.
The processes for manufacturing probe-storage devices envisage in a known way formation of the array of interaction systems 2 starting from an SOI (Silicon-On-Insulator) wafer, via micromachining techniques that envisage release of the various supporting elements 5 from an epitaxial layer of the SOI wafer, via appropriate chemical etching of an underlying oxide layer and if necessary of a bulk layer of the wafer. The interaction elements 6 are typically made prior to the step of release of the corresponding supporting elements 5. The array of interaction systems 2 is then coupled to a CMOS wafer (substrate 3) integrating the associated interface/control electronics by means of “chip-to-wafer” or “wafer-to-wafer” bonding techniques.
Known manufacturing processes have a number of problems, amongst which: high costs, mainly due to the use of composite SOI wafers; the need to resort to wafer-to-wafer bonding techniques to couple the interaction systems to the corresponding interface/control electronics; and the incompatibility with the so-called “CMOS back-end” (i.e. the formation of the MEMS structures after carrying out of the CMOS processes, in a same wafer of semiconductor material) due to thermal budget issues of the associated micromachining steps, and to recipe uniformity issues of silicon chemical etching. Furthermore, the steps of formation of the interaction elements 6 and release of the supporting elements 5 pose a series of problems of process integration, in particular for ensuring an adequate protection of the interaction elements 6 already formed, in process steps subsequent to their formation.
Up to now, fully satisfactory processes for manufacturing interaction systems for probe-storage devices have not been proposed.